This invention generally relates to vacuum surfaces, such as drums, plates, or surfaces of other configurations, which use suction to secure a workpiece or flexible media positioned over suction openings located on the vacuum surface, and more particularly to configurations that allow the vacuum surface to respond to the shape and placement of the workpiece by providing suction only suction openings under or near the workpiece to the vacuum. The invention further relates to the use of vacuum surfaces used to secure printed circuit boards during the manufacturing thereof.
Vacuum tables, vacuum plates and vacuum drums, collectively referred to herein as xe2x80x9cvacuum holders,xe2x80x9d are workpiece holding and restraining devices having a vacuum surface. A common approach for manufacturing such a vacuum holder is to have many suction openings terminating at a vacuum surface. The application of a vacuum produces a pressure difference across the workpiece, which it turn imparts a suction force on the workpiece towards the vacuum surface. The position, size and shape of the suction openings relative to the workpiece determine the required amount of vacuum and the suction force per workpiece area.
The uses for vacuum holders include a variety of industrial and commercial applications that require locating, restraining or transporting pieces. For vacuum plates and tables, workpieces are commonly positioned on the plate and the suction force is then engaged. Vacuum plates are used, for example, to transport a printed circuit board (PCB) from a stack to a direct imaging exposure device such as the exposure device described in co-assigned U.S. patent application Ser. No. 60/107,842. Vacuum drums are commonly configured as rotating cylindrical surfaces and are used for transporting flexible media, such as film and paper. Vacuum holders are used to position, secure and transport paper or film for scanning or printing/exposing. Paper processing and printing machines, for example, use vacuum holders to transport film or paper from one part of the machine to another. Other machines, such as imagesetters, laser printers and rotary data scanning and recording devices, use vacuum drums to support flexible media during exposure or scanning. In these devices the drum rotates with the suction engaged. Suction attaches the media to the drum at the point of first contact with open suction openings, and then pulls the media around the drum as it rotates.
All vacuum holders with vacuum surfaces incorporate a series of channels and suction openings in the vacuum plate or drum, allowing for one or a few vacuum connections to provide suction over an area of the surface. In many early and some contemporary prior art systems, the internal vacuum plumbing is configured so that suction is applied to all suction openings simultaneously. Another approach for manufacturing vacuum holders is to use a xe2x80x9cporousxe2x80x9d surface instead of suction openings. Regardless of the approach taken, there is a balance between the flow restriction of the surface and the vacuum source. A large suction force over a large area usually requires a low restricted porous material with a high capacity vacuum source.
There are several problems inherent in prior art configurations that result in a vacuum pumping requirement larger than the minimum needed to secure the workpiece. Consider the operation of a vacuum plate in which the workpiece is placed on the surface, covering some of the suction openings. Uncovered suction openings have a constant pumping requirement, and thus there will always be an excess capacity whose amount is determined by the minimum workpiece size. Covered suction openings will have a large pumping requirement until a vacuum seal between the workpiece and surface is formed, at which time the pumping requirement diminishes, theoretically approaching zero for a perfect vacuum seal. The pumping requirement will decreased from an initial value which must accommodate all of the covered suction openings, to nearly zero as a vacuum seal is formed. Thus it is seen that prior art vacuum holders require vacuum pumps that are oversized relative to the minimum capacity needed to restrain the workpiece.
Similar problems also occur in vacuum drum applications. The drum first makes contact with and picks up the leading edge of a flexible media. As the drum rotates, the media wraps about the dram and is held in place at the point of contact with the drum. In this application, the number of uncovered suction openings, and hence the pumping requirement, decreases as the rotation proceeds and suction openings are covered. The vacuum system must be capable of accommodating all of the initially uncovered suction openings.
Having several suction openings not covered by the workpiece may also produce undesirable noise and vibration.
Thus the application of suction simultaneously to all of suction openings on the vacuum surface produces several problems in earlier prior art systems. These can be characterized as requiring vacuum pumping overcapacity due to 1) uncovered suction openings, and 2) exposing all covered suction openings simultaneously. The problems due to uncovered suction openings has been previously acknowledged but only partially addressed in several U.S. Patents. Thus in U.S. Pat. No. 5,716,048, Morrisette describes a drum mask placed over the drum, where the mask is configured to cover those suction openings not covered by the media. This solution effectively tailors the vacuum drum to a media size as determined by the available masks. As noted in Morrisette, a mask must be produced for every media size, and the operator or machinery involved must adapt to changes in media size by changing masks. While that invention improves the performance by lowering the pumping requirement for each media size due to changes in the number of covered suction openings, this prior art invention requires intervention by either the operator or some machinery to choose the appropriate mask size. Furthermore, the suction force may be different for different masks because the area kept uncovered and number of free suction openings may differ.
Both U.S. Pat. Nos. 5,183,252 and 4,202,542 describe various methods for allowing vacuum drums to accommodate a few different media sizes through valving mechanisms that applying vacuum to pre-selected patterns of suction openings. These solutions do not require the additional mask hardware required by Morrisette, but do require complex, externally switchable vacuum plumbing if many different media sizes are to be accommodated. As with Morrisette, these references must also incorporate means to detect the size or orientation of the media. Each of these prior art solutions adapts the vacuum drum to a predetermined number of media sizes and orientations, and thus is not easily adaptable to sizes, shapes or orientations not considered in the initial machine design. In addition, none of the prior art addresses the excess pumping requirement due to applying suction to all of the suction openings simultaneously.
U.S. Pat. No. 5,374,021 to Kleinman includes a vacuum chamber which is divided into several sub-chambers each connected via a control passageway to one or more suction openings on a vacuum surface. Each control passageway includes a valve which is biased to keep the passageway open, and configured to close when the sub-chambers openings are not covered by a workpiece and a vacuum is applied to the vacuum sub-chambers. The valves of the passageways to openings that are covered by a workpiece remain open so that a vacuum is applied to hold the workpiece. The Kleinman system thus in effect provides a xe2x80x9cself adapting maskxe2x80x9d comprised of all the valves that are of the passageways to openings that are not covered by the workpiece. This offers advantages over the Morrisette and systems of U.S. Pat. Nos. 5,183,252 and 4,202,542 in that the Kleinman system adapts to all sizes, shapes or orientations.
The Kleinman system, however, still has several shortcomings. In addition, none of the prior art addresses the excess pumping requirement due to applying suction to all of the suction openings simultaneously.
This aspect of the present invention provides the benefit of limiting stresses on fragile workpieces.
An object of the present invention is providing a vacuum holder to restrain a workpiece with minimum or close to minimum vacuum pumping requirement.
Another feature of the present invention is providing a vacuum holder that can automatically adapt to a large number of workpiece sizes and orientations, using the same minimum or close to minimum vacuum pump requirement.
Yet another feature of the present invention is providing a vacuum holder that provides suction primarily to those suction openings covered by a workpiece.
Yet another feature of the present invention is providing a vacuum holder that can operate with a nearly constant, unregulated vacuum pumping requirement, independent of the workpiece size. Yet another feature of the present invention is providing a vacuum holder that is less bulky and less expensive as a result of decreased vacuum pumping requirements.
Yet another feature of the present invention is providing an adaptable vacuum holder that is both inexpensive and easily assembled.
Another feature of the present invention is the ability to reduce the stress and deformation of the workpiece through the slow, directional application of vacuum to the workpiece surface.
Another feature of the present invention is that it provides for holding media of different sizes with the same initial suction condition.
These and other features are provided for in an automatically adapting vacuum holder for supporting a workpiece through the application of a vacuum from a vacuum source, this vacuum holder comprising (a) a base having a workpiece support surface adapted for supporting a workpiece thereon, and (b) a vacuum plumbing system connected to the vacuum source through at least one vacuum port. The plumbing system includes (i) a plurality of chambers positioned along one or more directed lines of connection emanating from the vacuum port, each line of connection including a chamber directly coupled to one or more associated vacuum ports, (ii) a plurality of passageways positioned between any two chambers along any line of connection for controllably connecting each chamber along a line of connection to the next chamber further from the vacuum source along the line of connection, each passageway having a connected state and a disconnected state substantially connecting and substantially not connecting, respectively, the two chambers on either side thereof, and (iii) a plurality of vacuum bores each extending from the surface to a chamber to define a suction opening on the surface and configured to be substantially covered when a workpiece is placed thereon. Each chamber is either directly connected to the vacuum source or capable of being connected to the vacuum source via the passageways along a line of connection from the vacuum source. Each of the passageways from any particular chamber to the next chamber along any of the particular chamber""s lines of connection is biased to be in the disconnected state to the next chamber along any of the particular chamber""s lines of connection when the vacuum is not applied. Each of the passageways also is configured to remain in the disconnected state to the next chamber along any of the particular chamber""s lines of connection if the suction opening of the vacuum bore of the particular chamber is not covered by the workpiece. Each of the passageways also is configured to be in the connect state to the next chamber along any of the particular chamber""s lines of connection when the vacuum is applied and when the workpiece is placed on the surface so that the workpiece substantially covers the suction opening of the particular chamber and all the suction openings of the chambers closer to the vacuum source along any of the particular chamber""s line of connection. In this way, the vacuum holder supports the workpiece and limits the number of uncovered suction openings to which the vacuum source is coupled by sequentially opening suction openings along the lines of communication, thus automatically regulating the amount of vacuum necessary to restrain the workpiece.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.